Officials have said that the planes, at least initially, are unlikely to fly too close to the front-line fighting, meaning it is unclear they will even be able to deter attacking enemy aircraft from crossing into Ukraine from Russian airspace.

Belgium, Denmark, the Netherlands and Norway have pledged to provide Ukraine with 80 F-16s, but most of those won’t arrive for years. The United States, which approved the transfer of planes to Ukraine by NATO allies last summer after long blocking Kyiv’s request, has not provided or pledged any aircraft.

Officials declined to say exactly how many jets Kyiv will receive this year, but it won’t be more than one squadron — about 20 jets — and just six pilots were projected to complete training by this summer, because the program has limited spots and has been marred by delays.

The expected limited role of the U.S.-made planes highlights a deepening fault line between Kyiv and its Western supporters. U.S. officials had long said that the F-16s were unlikely to give Ukraine a decisive advantage because of Russia’s robust air defenses. Ukrainian officials, however, respond with a familiar refrain: With F-16s, as with other materiel, the West has supplied too little, too late.

President Volodymyr Zelensky has said Ukraine needs more than 100 F-16s to combat Russia’s huge air force and has called the quantity of jets Ukraine is receiving “insufficient.”

“The decision on the F-16 is strategic,” he said this month. “The number is not yet strategic.”

Some analysts are even more skeptical than Zelensky. Becca Wasser, an analyst at the Center for a New American Security, said the F-16s might be most potent as a psychological and morale boost for Ukrainians and a specter for Russian conscripts — in part because battlefield conditions have changed since last year when the decision to send the planes was announced.

“We’ve seen this pretty routine pattern when it comes to Western military equipment for Ukraine,” she said. “There’s an immediate need for them. Often by the time they are delivered that immediate operational need is null and void based on the rate of battlefield adaptation.”

But they may also fill urgent needs, Wasser said, because of their dual role of shooting down enemy aircraft and hitting ground targets.

So far, Western nations donating the fighter jets to Ukraine have not disclosed any restrictions on how they can be used. In other cases, Ukraine is limited in using Western weapons to strike targets in Russia. Officials have said Washington has confined Kyiv to firing less than 100 kilometers, about 62 miles, inside the border.

The question of restrictions might be moot because F-16s initially are expected to fly a cautious distance from the front, officials said.

A Ukrainian defense official, who, like others in this story, spoke on the condition of anonymity to discuss a sensitive security matter, said “dozens” of pilots were tapped for F-16 training. But limited instructors and training facilities have forced Ukraine to send one group of pilots and engineering staff at a time, the official said.

Four months of English lessons in Britain to learn key terminology precede working with the aircraft, making it a year-long process even for experienced pilots.

Ukraine also cannot afford to part with too many experienced pilots for such a long stint because they are needed for combat duty, the defense official said.

“The main restriction … is the amount of planes and time of training,” a second Ukrainian official said. “If Russia has 300 [fighters] and you have much less, you can’t operate properly.”

“We will not use it too close to the Russians” due to the threat of air defenses, the official said, adding that Ukraine also lacks some technical gear and additional training for maintenance engineers.

The Pentagon declined to answer questions about F-16s provided to Ukraine or Russia’s preparations to counter them.

In routine circumstances, F-16s carry a variety of weapons that allow them to target enemy aircraft and strike enemy positions on the ground.

But Col. Gen. Oleksandr Syrsky, Ukraine’s commander in chief, said in a recent interview with The Guardian that F-16s will fly at least 25 miles from the front line. Ukraine is unlikely to use the fighters to strike sites in Russia as a result.

The Ukrainian defense official said Kyiv’s F-16s will carry the AIM-120 Advanced Medium-Range Air-to-Air Missile — projectiles in a broader class of munitions fired by the NASAMS ground air defense systems Western countries have provided.

One missile variant has a range of about 100 miles, the official said — longer than several other aircraft-dropped munitions provided to Ukraine.

But the official said that Ukraine already has too few AIM-120 missiles and will have to split them between the F-16s and NASAMS.

Protecting the F-16s on the ground will also be a challenge given that Ukraine’s airfields are all within Russian missile range. Moscow recently struck several aircraft left exposed on runways.

The Ukrainian defense official said that “it’s not possible” for Kyiv to build covered concrete hangars to fully protect aircraft during wartime. Instead, Ukraine uses camouflaging techniques and even parks model planes on airfields as decoys.

A senior Ukrainian military official said Russia has been preparing for Ukraine to receive F-16s. Late last year, the official said, Russia launched several missiles with dummy warheads from an S-400 air defense system in Dzankoy, a town in occupied Crimea.

The projectiles reached the Ukrainian city of Kremenchuk, more than 200 miles away, the official said, adding that he suspected Moscow was testing the S-400’s range to shoot down aircraft.

Ukrainian officials and airmen have long bemoaned their existing, outdated fleet of MiG-29, Su-27 and Su-24 planes that have inferior radar range for target detection — about 25 miles — than Russia’s newer models of the same aircraft. The F-16’s radar can spot targets about 125 miles away, making them safer because pilots do not have to get as close to enemy planes to fire at them.

While the F-16 offers an improvement over Ukraine’s existing jets, it is still an older-generation fighter compared with Russia’s newer, more sophisticated air force. Ukraine’s military intelligence directorate previously reported that Moscow has more than 200 fighters, dwarfing Ukraine’s fleet.

As Ukraine’s air defenses have weakened in recent months, especially near the front, the Russians have flexed their air power in riskier combat missions. One of the most destructive weapons Russia uses, guided glide bombs, are nearly impossible for Ukrainian forces to shoot down once launched, officials have said. The only solution is to target the aircraft or their bases.

But officials said the threat of F-16s could scare some Russian aircraft from getting too close and dropping the bombs.

Ukraine has pleaded for fighter planes since early in Russia’s invasion, but the United States initially opposed the idea. In May 2023, under heavy pressure, the Biden administration reversed course.

Back then, Ukrainian officials hoped to get them in theater quickly, potentially in time for their 2023 counteroffensive, but it soon became clear that timeline was way off.

European nations stepped in to coordinate the training, led by the Netherlands and Denmark. At the recent NATO summit, the United States, the Netherlands and Denmark issued a statement confirming that “Ukraine will be flying operational F-16s this summer.”

Defenders of the plan say it will help even the scales with Russia, which has used close air support to advance in Ukraine.

When you can “actually call in close support from overhead, that gives you significant advantage, and an ability to actually move a battle line, a battle front, forward,” said a NATO official, speaking on the condition of anonymity to discuss battlefield assessments. “So the arrival of fighter aircraft certainly gives Ukraine more capability.”

But there are long-standing doubts about overall impact. The United States especially never seemed enthusiastic, first arguing that there were other priorities and more recently that there will not be enough pilots and support to use the planes effectively.

“The training pipeline is pretty meager,” a senior Defense Department official said last month, speaking on the condition of anonymity to brief the press.

Michelle “Mace” Curran, a former F-16 pilot, said the plane is an upgrade compared with Ukraine’s existing Soviet-era models. Its sensors and missiles can track and fire at targets pilots cannot see with their own eyes, allowing them to stay further back to avoid enemy fire.

One challenge for Ukraine is the F-16’s complexity, Curran said, which requires a deep maintenance regimen involving specialists focused on engines, fuel and avionics.

Curran was once assigned to the Thunderbirds demonstration team, which brings eight stripped-down jets to its shows. About 90 of the 130-person team are maintenance crew, Curran said, underscoring the upkeep required.

“I think people are waiting for this climactic moment of the jets arrive and everything shifts, and it just doesn’t work like that,” Curran said. “They’re complicated. They’re complex to support and to operate. It’s exciting, but we have to be a little bit patient to see the results.”

There is a chance the F-16s will follow the path of U.S.-provided Abrams tanks, said Wasser, the defense analyst. A limited number arrived after conditions had changed and had debatable impact on last year’s counteroffensive. Still, it is possible that the F-16s will address Ukraine’s need for better air defenses, she said.

It makes sense to use the planes conservatively, Wasser said. “If you have this exquisite capability, are you going to use it immediately, knowing that there’s a greater risk of it being expended?” she said. “Or are you going to withhold it and use it so it has broader strategic value?”

Horton reported from Washington and Rauhala from Brussels. Anastacia Galouchka contributed to this report.